1. Field of the invention
The present invention generally relates to a method of producing, in a workpiece, a groove extending along a curve such as a spirally extending curve by using a milling cutter, e.g., an end mill. The curved groove is intended for use in receiving a mechanical seal. More particularly, the present invention relates to a method of cutting a curved groove in a metallic workpiece, adapted for forming an involute-curve groove in a spiral portion of a scroll member which is an indispensable element of scroll-type fluid machines including fluid compressors and fluid expanders.
2. Description of the Related Art
A conventional scroll member of a scroll-type fluid compressor has a general shape as shown in FIGS. 12 through 14. Namely, the scroll-type compressor has a stationary scroll member 1 fixedly attached to a housing (not shown) of the compressor, and a movable scroll member 4 movably engaged with the stationary scroll member 1. The stationary scroll member 1 includes a disk-like base plate 2 made of a metallic material such as aluminum, and a metallic spiral member 3 formed to be integral with the base plate 2. The movable scroll member 4 also includes a disk-like base plate 5 made of a metallic material such as aluminum, and a spiral member 6 formed to be integral with the base plate 5. When the stationary and movable scroll members 1 and 4 are assembled in the scroll-type compressor, the spiral members 3 and 6 are engaged to be angularly shifted by 180.degree. from one another, and the side walls of the two spiral members 3 and 6 are kept in contact with each other at a plurality portions of the two members 3 and 6. Further, the axial ends of the respective spiral members 3 and 6, which extend spirally, are arranged to be close to an inner face of the engaged base plates 5 and 2 of the scroll members 4 and 1.
Thus, when the scroll-type compressor is in operation, and when the movable scroll member 4 orbits with respect to the stationary scroll member 1, compression chambers 7 formed by the engaged scroll members 1 and 4 gradually move from the outer portions of the spiral members 3 and 6 toward the central portion of the respective spiral members 3 and 6 so as to reduce the respective volumes of the compression chambers 7. Thus, in the respective compression chambers, a refrigerant gas for a refrigerating system is compressed, and is discharged as a compressed gas from the respective compression chambers 7 to the external refrigerating system through a central discharge port 8 of the compressor.
As best shown in FIG. 14, the respective spiral members 3 and 6 of the scroll elements 1 and 4 are provided with end faces in which a spirally extending groove 9 is formed to receive a mechanical seal 10 made of a synthetic resin material having high abrasion durability and high temperature-resistant property. The mechanical seal 10 of the respective spiral members 3 and 6 are constantly in tight contact with the inner faces of the engaged base plates 5 and 2 of the scroll members 4 and 1, so that the compression chambers 7 are fluidly isolated from one another by the mechanical seals 10 during the operation of the compressor.
In order to obtain the good sealing function of the mechanical seals 10 of the respective scroll elements 1 and 4, the dimensional accuracy of the mechanical seal 10 must be maintained during the manufacture thereof. Further, the dimension of the groove 9, especially, that of the width of the groove 9 of the respective scroll members 1 and 4 must be very accurate so as to snugly receive the mechanical seal 10 without any play between the side walls of the groove 9 and the mechanical seal 10.
The groove 9 of the respective scroll elements 1 and 4 extends so as to conform to the shape of the curve of the spiral members 3 and 6, i.e., the shape of an involute curve, and therefore, the production of the groove 9 is ordinarily achieved by the method of cutting by using a milling cutter, typically an end mill 20, as shown in FIG. 2.
Nevertheless, when the groove 9 of the respective spiral members 3 and 6 of the scroll elements 1 and 4 is cut by the end mill 20, a problem as described hereinafter occurs. Namely, when the cutting of the groove 9 from an end thereof to the other end thereof is continuously processed by only one way movement of the end mill 20 along the involute curve, the end mill 20 cuts one of the side walls of the groove 9 in up milling in which a workpiece 21 (the scroll element 1 or 4) is relatively fed in a direction "A" and the end mill 20 is rotated in a direction "B" against the direction "A" as shown in FIG. 5A, and the end mill 20 cuts the other side wall of the groove 9 in down milling in which the workpiece 21 (the scroll element 1 or 4) is relatively fed in a direction "A" and the end mill 20 is rotated in a direction "B" corresponding to the direction "A" as shown in FIG. 5B. In FIGS. 5A and 5B, the reference numeral 22 designates the shape of a chip produced during the cutting operation of the end mill 20.
When up milling is compared with down milling, it can be seen that in the up milling process, the entrance of the cutting edge of the end mill 20 into the workpiece 21 gradually increases from a minimum to a maximum, as shown in FIG. 5A, and therefore, when the rotation of the end mill 20 and the relative workpiece feed are at high speed, the cutting edges of the end mill 20 often fails to enter the workpiece 21 by slipping thereof on the surface of the workpiece 21, resulting in a formation of a coarse or ridged surface on the side wall of the groove 9.
Further, for example, when the workpiece 21 is a soft aluminum workpiece, the workpiece 21 is easily elastically deformed during the cutting operation of the end mill 20. Thus, the elastic deformation of the workpiece 21 makes it more difficult for the cutting edges of the end mill 20 to enter the workpiece 21.
On the other hand, in the down milling process of the other side wall of the groove 9, the initial entrance of the cutting edge of the end mill 20 into the workpiece 21, i.e., the side wall of the groove 9 is surely accomplished so that removal of the workpiece 21 decreases from a maximum to a minimum, as shown in FIG. 5B. Namely, the cutting of the workpiece 21 is smoothly processed without the formation of any appreciable coarse or rough surface on the side wall of the groove 9. The formation of the rough or coarse surface on one of the side walls of the groove 9 of the respective spiral members 3 and 6 degrades the surface finish of the side walls of the groove 9 while bringing about an reduction in the dimensional accuracy of the width of the groove 9. Accordingly, the mechanical seal 10 cannot be snugly fitted in the groove 9.
Taking into account the above-mentioned situation in the formation of the groove 9 by using the milling cutter, it will be easily understood that the cutting of both side walls of the groove 9 of the respective spiral members 3 and 6 of the scroll elements 1 and 4 should be processed in the down milling by relatively feeding the milling cutter (the end mill 20) in both ways (in the directions "A" and "B") along the predetermined curve, e.g., the involute curve. Nevertheless, the feeding of the milling cutter with respect to the workpiece 21 in both ways (forward and return feeding) causes an increase in the cutting time of the groove 9.
Further, if the production of the groove 9 is carried out by providing a relative feed between the milling cutter, i.e., the end mill 20 and the workpiece 21 at a reduced speed in only one way, in order to reduce the formation of the coarse and rough surface on the side walls of the groove 9 of the spiral members 3 and 6, the cutting time of the groove 9 increases bringing about a reduction in the production rate of the scroll elements 1 and 4.